Step-up converters, also referred to as boost converters, convert an input voltage U1 into a higher output voltage U2. They are used in many battery-powered devices in which the electronics require a voltage that is higher than the battery voltage, for example in notebooks, mobile telephones, or various household appliances. A step-up converter from the related art is shown in FIG. 1. Step-up converter 1 shown there includes an inductor L, a diode Di, a capacitor C, and a switch S which is controlled in a predetermined clock pulse.
Step-up converters are generally connected in a control loop within which and by which the operation of the step-up converter is controlled. So-called digital controllers, with the aid of which the operation of the step-up converter in the control loop may be precisely adjusted and controlled, are an important element within these control loops nowadays. The path of the control loop is modelled in designing the control loop for a step-up converter. The so-called path transmission function of a step-up converter to be determined within the scope of the modeling is a function of various parameters of the control loop. These parameters include, among others, for example the load resistance that is fed in the path of the control loop by the step-up converter, the output and input voltages, and the LC filter values of the output converter. This results in various path transmission functions for various parameter values, and for these various path transmission functions, various optimal configurations of the above-mentioned controllers. Each controller configuration results from the specific setting of various coefficients of the controller, so that two different controller configurations differ from one another in the different setting of at least one controller coefficient.
Of all the (above-mentioned) parameters whose change can affect the path transmission function of a control loop, changes in the load resistance have the greatest effects on the path transmission function of the control loop of a step-up converter. The path transmission function of a step-up converter has a so-called right-half plane zero (RHPZ) point in the pole-zero point diagram, which has adverse effects on the stability of the control loop of the step-up converter. For this reason, transition frequency fT of the control loop for controlling step-up converters in the related art is selected to be significantly below the frequency that is associatable with the right-half plane zero point. However, the frequency that is associatable with the right-half plane zero point migrates to higher frequencies with increasing load resistance, which would allow transition frequency fT of the control loop to be selected to be higher for higher load resistances than for lower load resistances, thus allowing a faster control loop to be implemented.
However, in the control loops for step-up converters of the related art, a single, largely unchangeable controller configuration is generally selected, and consequently a transition frequency fT is set which also remains constant when there is a change in the load situation in the path of the control loop. Invariable transition frequency fT which is preset in the controller by an appropriate choice of the controller coefficients is specifically selected in such a way that a worst-case condition—the presence of the smallest possible load resistance RL in the path of the control loop—is always covered. However, as a result of this procedure, transition frequency fT is constantly low, independently of load resistance RL, and the control loop is therefore generally slow.
FIG. 2 graphically illustrates this relationship. In particular, FIG. 2 shows a diagram in which the frequency that is associatable in each case with a particular right-half plane zero point RHPZ at a given load resistance RL in the control loop, as well as various transit frequencies fTmax, fTinst of the control loop of a step-up converter, are plotted in Hertz as a function of load resistance RL of the path of the control loop in ohms. The straight line denoted by reference character RHPZ in FIG. 2 thus shows the frequency that is associatable in each case with a right-half plane zero point of the control loop at a given load resistance RL, and that increases with increasing load resistance RL in the path of the control loop. The straight line denoted by reference character fTmax shows the maximum allowable transition frequency, at a given load resistance RL, at which the control loop may be reliably, stably, and optimally operated. The constant denoted by reference character fTinst shows the preset, unchangeable transition frequency in the controller according to the related art, so that the gray shaded area A in FIG. 2 depicts an unutilized potential of the control loop.